The invention relates to a device for measuring an oxygen content in gases. The measuring device has two configurations that enclose a gap between them and form a magnetic field. A sensor is held rotatably in the gap and can be deflected out of its position of rest as a function of the concentration of oxygen.
Such a device is used, for example, in chemical process technology.
Published, Non-Prosecuted German Patent Application DE 230 18 25 A discloses a method and a device which make use of the paramagnetic properties of oxygen in order to measure its concentration in a gas mixture. For this purpose, two magnets are disposed at a distance from one another in such a way that a gap of a defined size remains between them. Due to the magnetic field that forms in the gap, the oxygen is concentrated there. A measuring device with a sensor is disposed within the gap. The sensor is in the form of a dumbbell. It is held in such a way that it can be rotated about an axis that lies at its center of gravity. The sensor is moreover held in the gap in such a way that its longitudinal axis is oriented parallel to a longitudinal axis of the gap. The sensor is forced out of the gap when the oxygen concentration within the gap is increased in relation to the surroundings. The resulting deflection of the sensor is a measure of the oxygen concentration. With the aid of a mirror attached to the sensor and of a light balance disposed outside the gap, a highly accurate detection of the position of the sensor is possible. The sensor surface has disposed on it a coil that is led all around the edge of the sensor. With the aid of a current conducted through the coil, a restoring force can be generated, by which the sensor can be brought back into its position of rest again. The light balance determines when the position of rest has been reached. The magnitude of the current necessary for generating the restoring force is utilized in order to determine the oxygen concentration. There is a linear relation between the magnitude of the current and the oxygen concentration in the gap. A further measurement value that is used is the magnitude of the current that is necessary in order to bring the sensor out of the deflected position back into the position of rest when the gas mixture does not contain any oxygen.
The response times of the device are very long, since the inertia of the sensor is very high due to its structural configuration. Since the entire set-up of the device is large, the quantity of gas in the measurement chamber in which the device is disposed is also large. The exchange of the gas lasts for a correspondingly long time. Nor can it be sped up by a higher flow velocity, since this influences the deflection of the sensor and therefore leads to measurement errors.
It is accordingly an object of the invention to provide a device for measuring the oxygen concentration in gases which overcomes the above-mentioned disadvantages of the prior art devices of this general type, which has a minimized set-up and short response times.
With the foregoing and other objects in view there is provided, in accordance with the invention, a measuring device for determining the oxygen content in a gas. The measuring device has two devices enclosing a gap between them and forming a magnetic field. A frame is disposed in the gap and at least one sensor is held rotatably and moveably in the gap by the frame. The sensor can be deflected out of a position of rest in dependence on a concentration of the oxygen. The measuring device according to the invention is provided with the sensor that has small dimensions and which is held rotatably within the frame. The frame is disposed within the gap formed between the two configurations, by which a magnetic field can be generated in the gap. The sensor may be formed by a parallelepipedal body alone or by a parallelepipedal body to which a web-like structural element is fastened. Each of the parallelepipedal bodies is either hollow or solid. Preferably, the sensor is in the form of a dumbbell and is formed by two parallelepipedal bodies that are connected to one another via a web-shaped structural element. The web-shaped structural element is held rotatably in the frame centrally via two holding elements. The sensor, the frame and the holding elements are manufactured preferably from silicon. It is consequently possible to produce all the structural elements by etching techniques. Cost-effective mass production of the sensor becomes possible, using photolithography for structuring the etching mask.
At the same time, very narrow tolerances in the sensor properties, such as sensitivity and offset, can be achieved. To protect the silicon against corrosive gases, all the surfaces that are in contact with the gas atmosphere are coated with protective layers, such as silicon carbide, silicon nitride or silicon oxide.
The sensor is held by the resilient holding elements. The resilient holding elements are connected to the frame in such a way that they make it possible for the sensor to rotate about its mid-axis. The dimensions of the sensor are greatly reduced, as compared with known structures. The miniaturization of the sensor makes it possible to reduce the measurement gas volume. The exchange of the measurement gas can thereby take place more quickly, even when the flow velocity is reduced. This allows shorter response times and the reduction in the measurement errors, since disturbances due to the action of the flowing gases on the sensor are ruled out virtually completely. If the response times need to meet only reduced requirements, the application of current to the sensor may be dispensed with completely. Gas exchange then takes place by diffusion, without any adverse influence on the sensor.
By the decrease in the sensor height, the distance between the configurations generating the magnetic field can be reduced. As a result, the magnetic field strength in the gap is increased, and therefore a greater enrichment of oxygen within the gap is also achieved, so that the sensitivity of the sensor is appreciably improved. At least one conductor track is formed on the surface of the sensor. A current of variable magnitude can be conducted through the conductor track. With the aid of the current, a force is generated which is necessary for compensating the deflection of the sensor that the latter experiences in the magnetic field due to the concentration of oxygen. The conductor track is structured on the sensor by vapor deposition and etching methods.
In accordance with an added feature of the invention, at least one of the two boundary surfaces of the frame running parallel to one another has an orifice formed therein. The sensor has a region with a reflecting coating disposed thereon, and through the orifice electromagnetic radiation can be conducted onto the reflecting coating of the sensor.
In accordance with an additional feature of the invention, the sensor, the frame and the resilient holding elements are manufactured from silicon.
In accordance with another feature of the invention, a current source generating a current is provided. A conductor track is connected to the current source and is disposed on a surface of the sensor, and in that a restoring force of a defined magnitude acting on the sensor can be generated by the current conducted through the conductor track. In accordance with a further feature of the invention, the parallelepipedal bodies have a wall thickness and the sensor has a moment of inertia able to be set by the wall thickness of the parallelepipedic bodies.
In accordance with a further added feature of the invention, a deflection of the sensor can be limited to a defined value by geometric dimensions of the resilient holding elements.
In accordance with a concomitant feature of the invention, the sensor has a length of 2 to 15 mm, a width of 0.5 to 3 mm and a height of 0.1 to 2 mm, and the frame has an inner area of between 1.5 mm2 and 50 mm2.
Other features which are considered as characteristic for the invention are set forth in the appended claims.
Although the invention is illustrated and described herein as embodied in a device for measuring the oxygen concentration in gases, it is nevertheless not intended to be limited to the details shown, since various modifications and structural changes may be made therein without departing from the spirit of the invention and within the scope and range of equivalents of the claims.
The construction and method of operation of the invention, however, together with additional objects and advantages thereof will be best understood from the following description of specific embodiments when read in connection with the accompanying drawings.